It has as its object a method for quantitative determination of viscoelastic properties of a complex and generally opaque medium, based on the calculation of the movement of particles in suspension starting from the detection of the light scattered by the medium. A device that is designed for the implementation of this method is also the object of the invention.
It is very useful for manufacturers to characterize—from the standpoint of their rheological properties—the products that they are developing and whose quality they are monitoring, for example for following the evolution over time of the viscosity of a formulation or for comparing the viscoelastic properties of two products of close formulations. To do this, viscometers and rheometers with rotating and oscillating moving bodies that measure the resistance to shear and to flow are widely used and make it possible to calculate the corresponding viscosity. The principle is based on the measurement of the force that is necessary to the rotating or oscillating of a gauge that is in contact with the product, with the gauge that can be a rod immersed in the product, or else flat or conical plates between which a film of the product to be tested has been spread.
In all of the cases, a sample is placed in contact with the gauge that is to be cleaned between each measurement, which represents a significant loss of time and introduces risks of misaligning or even degrading the device, and finally a lack of reliability of the measurements. The tested sample can only be used once. However, when the evolution of a composition over time is studied, or when it is desired to compare several formulas with one another, it is preferable to preserve the same sample for making (or redoing) all of the necessary measurements, so as to reduce as much as possible the possible variations of the composition of the products that are prepared and those that are inherent to human manipulations. Incidentally, these devices can be used only by individuals who have a high level of expertise, knowing to calibrate them and to implement them correctly to obtain reliable results. Routine use under production conditions is therefore virtually impossible.
To prevent these problems, a method that is based on optical measurements has been sought. It is actually known to determine certain properties that depend on the rheology of a complex medium by the measurement of the kinetics for movement of particles of a known size in suspension in a medium. The Brownian motion of these particles corresponds to trajectories whose characteristics are strongly tied to rheological properties and is analyzed by an elastic component (preservative part) and a viscous component (dissipative part).
The prior art teaches the possibility of following the trajectory of Brownian particles by detecting the time change of the speckle field produced by a laser wave that diffuses into the medium under study, using an optical technique that has been known for about 30 years under the name of DWS (Diffusing Wave Spectroscopy). The speckle grains are light interferences produced by the superposition of light rays that have travelled different paths in the medium. The method is based on the detection of the intensity variation of the light received by a single detector or by the pixels of a video camera (by scattering or by back-scattering according to the selected technique). The dynamics of the speckle field makes it possible to draw conclusions about the dynamics of the particles that have contributed to the scattering of the light.
If the number of time measurements of the light intensity is adequate, it is possible to derive a time auto-correlation function (g2). The use of a video camera as a detector offers the possibility of acquiring many more signals during the same time period and therefore makes it possible to obtain an auto-correlation of good quality in much less time by exploiting the ergodicity property that establishes that it is equivalent to averaging signals in various positions in space and averaging signals that are acquired in successive series over time. The prior art also described an alternative solution that is based on the use as a detector of a double cell (EP1720000).
In the case where a camera is used, the applicant advantageously introduced a more practical method for calculating the decorrelation of the speckle, by quantification of the inter-image distance, denoted d2 (as presented in detail in WO 2005/031324). The inter-image distance represents the movement of the particles over the time period that separates two images. This method makes it possible to access the movement dynamics of particles, a value that gives information in particular on the drying rate (or the loss of fluidity) of a mixture.
It does not make it possible, however, to attain the different components that account for rheological properties, namely the elastic module G′(ω) and the viscous module G″(ω) of the medium, whereby this distinction is often valuable and even essential for characterizing and predicting the behavior of a mixture.
Prior studies in this field have described the principle of the determination of the viscous and elastic moduli in two stages. The first stage makes it possible to pass from the optical signal to the movement of particles over time (kinetics of movement of the particles, formalized by the Déplacement Quadratique Moyen (DQM or MSD for Mean Square Displacement in English), by means of a suitable mathematical modeling and a certain number of approximations (Weitz—1993). The second stage (Mason—1995) is based on the Stokes-Einstein Law that is generalized for deducing from this kinetics the viscous and elastic properties of the sample, i.e., the value of G′(ω) or G″(ω).
However, the fluids targeted here are complex fluids and are thus characterized by the numerous parameters that are involved in taking their properties into account, unlike simple fluids whose behavior can be easily modeled. In particular, the models that are established for connecting optical measurements to physical properties of the media being studied involve the statistical distribution P(s) of the lengths of paths of photons between the light source and the receiver (between the laser and the camera). However, the direct calculation of the function P(s) in practice is possible only for simple cases, using semi-completed flasks or blades. Digital simulations could be applied for calculating the function P(s) of the complex fluids, but this type of solution can no longer be considered within an industrial context, because it is impossible to control all of the optical parameters and the laws that describe them. The calculation periods are inconsistent for offering a quick measurement result to the user, and their final precision is far from being ensured. These simulations have shown in particular that the greatest source of inaccuracy was the lack of knowledge of the parameters that influence the first of the two stages, i.e., the relationship existing between the inter-image distance d2 and the actual movement of the MSD particles.